The present invention relates to optical transport systems, and more particularly to dense wavelength division multiplexing (DWDM) optical transport systems.
Background: Optical Transport Systems
Consumer and business demand for data transmission capability has increased beyond all expectations with the introduction of the internet. Now, consumers may tap into an information base that literally spans the globe and contains more data than any other single information source. The internet also allows users access to music, video, and other entertainment. Likewise, businesses are now interconnected to a degree previously unheard of. However, the bandwidth of the various transmission media used to transport information limits consumer and business access. Government and industry regulation of airwaves combined with technological limits in data transmission has fostered innovation in telecommunications seeking to expand transmission capability in an effort to meet the ever growing demand.
Fiber optic transport systems is one of the promising technologies for increasing data transmission speeds and capacity. Fiber optic lines have many advantages over other media, and relatively recent gains make future use of this technology very attractive to both the buyers and sellers of bandwidth.
Fiber optic systems are bit-rate and format independent. Each of the multiple signals on a fiber can therefore be carried at a different rate and in a different format. For example, a DWDM (dense wavelength division multiplexing) network can transport different signals operating at OC-48 (2.5 Gb/s) and at OC-192 (10 Gb/s) simultaneously. This ability provides open ended growth potential for optical networks, allowing higher bit rate optical carriers (40 Gb/s and 160 Gb/s) to be implemented on existing fiber.
DWDM increases the capacity of embedded fiber by assigning incoming signals to specific frequency (or wavelength) bands within the bandwidth of the transmission. These signals are each modulated onto a light signal. The resulting light signals are then combined into one complex light signal onto one fiber. A given fiber will therefore have multiple channels transmitting simultaneously at different wavelengths and different bit rates, and possibly in different formats.
Several different wavelengths of light are used for optical transport systems. Silica-based fibers have particular characteristics, such as attenuation troughs, that make some wavelengths better choices than others. For example, sources emitting at a wavelength of 1550 nm provide the best attenuation characteristics (about 0.2 dB/km). The light emitted is concentrated at this peak wavelength. This signal is then dispersed over a free spectral range of about 32 nm (from 1530 nm to about 1562 nm). This range is broken into a number of channels, each channel occupying a channel spacing or spectrum line width of about 0.5 nm, corresponding to a particular bit rate. Most systems today operate in the C-band, or conventional band.
All current DWDM transport systems are designed with fixed channel spacings, meaning the hardware used does not allow channel spacings to be varied. Emitters are tuned to emit carrier frequencies corresponding to the central wavelength of a particular channel spacing within the system. These carrier signals are modulated to carry information, which could be video, voice, or any other type of information. Modulation increases the bandwidth of the signal in proportion to the amount of information transmitted. Higher bit rate signals require greater bandwidth channels. Filters within the DWDM system filter the modulated signals so that only frequencies within that channel's bandwidth may be coupled to the fiber. This eliminates noise and reflection between channels. Each emitter operates on a single channel, and the filters are of fixed bandwidth, which means that each channel in the DWDM system can only accommodate bit rates that require no more bandwidth than that channel's filter can pass. If more information is attempted to be modulated onto a channel than that channel is designed to carry, the bandwidth after modulation would be too great to pass the filters. Changing the bit rates of the channels currently requires changes in hardware.
As a simplified example, a system might be designed for two OC-48 carriers, one OC-192 carrier, and one OC-768 carrier. The channel spacings and spectrum widths of such a system are shown in FIG. 1. These channel spacings are fixed. For instance, the OC-48 carriers would occupy channels with a frequency bandwidth of 50 GHz centered on wavelengths λ1 and λ2. The OC-192 carrier would occupy a frequency bandwidth of 100 GHz centered at another wavelength, λ3. The OC-768 carrier would occupy a frequency bandwidth of 200 GHz centered at wavelength λ4.
FIG. 2 shows a simplified hardware configuration for a fixed channel spacing system 200. The signals are combined by combiners 202 and coupled into the transmission fiber 204. This system could handle two transmissions of 2.5 Gb/s (the two OC-48 lines), one transmission of 10 Gb/s (OC-192), and one transmission of 40 Gb/s (OC-768). These channel spacings are fixed, both in their central wavelength and in spectrum width.
Because the channels are of fixed spacing, the total bandwidth may not be used efficiently in many situations. For example, if the smaller bit rate transmissions are transmitted on higher bit rate channels then the full capacity of the system is not used. If such a system needed to accommodate three 2.5 Gb/s transmissions and one 40 Gb/s transmission, the OC-192 channel would be used for a 2.5 Gb/s transmission. This causes the smaller spectrum transmission to occupy unnecessary bandwidth in the DWDM system. A diagram of the channel spacing distribution in such a circumstance is shown in FIG. 3. The 2.5 Gb/s transmission on λ3 does not occupy the full spectrum width allocated to that channel, because that channel is designed to accommodate a 10 Gb/s transmission. This results in unnecessarily unused bandwidth.
Another circumstance where fixed channel spacing wastes bandwidth occurs when there is sufficient unused bandwidth on the fiber to accommodate a high bit rate signal, but that unused bandwidth is already distributed among several fixed, lower bit rate channels. In this case, the high bit rate signal may not be transmitted, and fiber capacity is wasted.
Fiber optic communications would therefore greatly benefit from a system that decreased the fiber waste from low bit rate signals occupying high bit rate channels, and allowed multiple low bit rate channels to accommodate a high bit rate signal.
Transport System with Tunable Channel Spacing DWDM
The present invention improves on all current optical transport systems by allowing the various channel spacings on a fiber to be tunable in either bandwidth, central wavelength, or both.
In the preferred embodiment, tunable filters are used to vary the spectrum width of channel spacings. Since the widths of channel spacings are variable, different amounts of data may be modulated onto a given carrier signal within a given channel, and the bandwidth allocated to that channel can be dynamically changed to accommodate the increase or decrease in bit rate. Tuning the widths of channels in this way allows for the most efficient use of system bandwidth by allocating narrow bandwidth channels to low bit rate signals and wider bandwidth channels to higher bit rate signals.
For example, if the required bit rate for a signal changes due to an increase or decrease in traffic, the channel spectrum widths may be dynamically varied for the most efficient use of the fiber bandwidth. Multiple lower bit rate channel spacings may be combined into a single higher bit rate channel spacing by simply increasing the passband of one of the tunable filters to include the passbands of the combined low bit rate channel spacings. In this case, the now “unused” low bit rate channels that were absorbed are turned off.
Conversely, if the traffic on a high bandwidth channel decreases, that channel's spectrum width can be tuned to a narrower width by, for example, decreasing the passband of the necessary filter. This would free up bandwidth for other signals, allowing the most efficient use of the fiber transmission bandwidth.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:                allows mixed variable bit rates on a single fiber;        combining lower bit rate channel spacings to create higher bit rate channel spacings;        divide higher bit rate channel spacings to create lower bit rate channel spacings.        